In mining operations explosives are loaded into boreholes. The explosive is detonated by means of a detonator positioned in the explosive. Ideally, the boreholes are primed and initiated at or towards a bottom/closed end of the borehole. In the case of bulk loaded explosives such as emulsion based explosives and ANFO, primers or boosters with an embedded or attached detonator are lowered into the borehole and pulled slightly off the bottom end of the borehole (out of the drill cutting or mud). The primary explosives are then loaded into the borehole.
At the bottom end of the borehole, gasses are confined enabling an explosive column provided in the borehole to do the most work when the explosives are detonated, before equalizing to ambient conditions. The placement of primers or boosters in a borehole accordingly plays an important role in the results obtained from the explosion.
Boosters or primers are cap-sensitive explosives typically packaged in cylindrical form. They are made of high velocity, high energy explosive material that has the capability to detonate commonly used bulk/primary explosives in the mining industry. By being cap-sensitive, they can be initiated themselves by lower energy detonators activated through a downline (such as detonator cord or shock tube). The primer or booster must have sufficient energy to initiate the detonation reaction in the explosive column and sustain it until the primed explosive produces enough energy to support the detonation reaction by itself.
It is known and proven through numerous field and laboratory tests that when a borehole, containing water, is bottom-pumped by primary explosives, the booster that was initially lowered to the bottom of the borehole by a downline, is pushed upward in the borehole by the displaced water initially and further by the rising explosive column, thus removing the primer or booster from the preferred location near the bottom end of the borehole. There is no control under these conditions as to how far the primer or booster will be lifted up the explosive column, but it is generally accepted that it would end up at the upper third of the explosive column. With the primer or booster now located at the top third of the explosive column, the initiation of the explosive column occurs near the top of the borehole. This occurrence is known as top priming.
When an explosive-filled borehole is top primed, the top of the borehole explodes, releasing gasses and often causing air blast or possibly the production of dangerous flying rocks. There is also a danger of losing the explosive confinement at the top of the borehole, thus reducing the effectiveness of the explosive column in breaking the rocks and negatively affecting the fragmentation produced by the blast.
One known solution which tries to address this problem is by making use of a weight (e.g. a brick) that is provided with a central hole that allows for a string to pass through the hole and secure the brick to a primer or booster by attaching the string to the booster. Another known solution is by making use of a small bag made out of netting material which is filled with small rocks to provide a weight which is sufficient to keep it at the bottom of a water-containing borehole. The bag is attached to the primer or booster by means of a string. Both of these solutions are impractical since they are slow to put together in the field and heavy to carry with the booster to every borehole on a mining bench that may typically have 100 to 250 boreholes (even as high as 500 boreholes).
A further problem of which the Inventors are aware is that a booster lowered into a borehole tends to end up against a side wall of the borehole due to the fact that the operator that lowers the booster, stands at a side of the borehole thus resulting in the downline, via which the booster is lowered, travelling over an upper edge of the borehole opening. The booster is therefore lowered along a sidewall of the borehole. In this position, an operatively lateral booster surface in close proximity with the sidewall will not be sufficiently surrounded by primary explosive materials and as a result part of the explosion's released energy will be utilized to break rock rather than initiating the explosive column.
The communication between an electronic detonator located in the borehole (embedded in the booster) and its control centre or blasting machine which is located on the surface, is accomplished through a very thin downline. Currently, mines are experiencing a high number of breaks in communication between electronic detonators and their control centres.
It is understood that the breaks in communication are mainly caused by the tension that the downline is subjected to, due to the fact that the booster and detonator are hanging at the end of the downline, and are pulled towards the bottom end of the borehole by the settling of the explosive column that was placed in the borehole after the booster and electronic detonator were placed in the borehole.
The settling of the explosive column inside the borehole and the consequential pulling and tensioning of the downline can result in a communication break in the downline due to stemming material, consisting of drill cuttings that may in turn contain small and sharp rocks or crushed rocks, that is placed/positioned above the explosive column and surrounds the downline. The stemming material may cut or crimp the high tension downline to the point of damaging its integrity and cutting off the communication line between the electronic detonator at the bottom of the borehole and the control centre at the surface.
Furthermore, the downline connected to the detonator is threaded from top to bottom through an open-ended tunnel-like hole in the booster and then pushed back up into a closed-ended parallel tunnel-like hole. This arrangement creates a U-shaped turn in the downline at the bottom of the booster which is made of hard non-flexible material. Due to the settling of the explosive column, the hanging booster will be pulled down towards the bottom end of the borehole. The net effect is that the downline may suffer a crimping effect at the U-shaped turn area at the bottom of the booster. This crimping effect has the potential of squeezing the downline to the point of cutting off the communication from the control centre to the detonator.
The above scenario is likely to be encountered in dry holes where straight ANFO is utilized, as well as water filled holes loaded with non-pumpable explosive blends such as a ratio of emulsion: ANFO of up to 50:50. It is not likely to be encountered in water filled holes that are pumped and that have an emulsion: ANFO ratio of 60:40 and above. This is due to the fact that the booster/detonator combination will more than likely float upwards towards the top of the borehole, thus reducing tension in the downline.
It is an object of this invention to provide means which the Inventors believe will at least alleviate at least some of these problems.